One example of in-vehicle noises generated inside a vehicle is a “muffled sound”. This sound is generated by a vibratory force that is generated by an engine rotation and is transmitted to a vehicle interior via a vehicle body or by cavity resonance in the vehicle interior. Further, another example of the in-vehicle noises is a “load noise”. This noise is a vibration that is excited by unevenness of a road surface during traveling and is transmitted to the vehicle interior as a noise. In order to reduce such an uncomfortable in-vehicle noise, a noise reduction device that forms a control sound having opposite phase and equal amplitude with respect to the noise, and cancels the noise through the control sound output from a speaker is known.
Such noise reduction devices are disclosed in PTLs 1, 2, and 3. These noise reduction devices use a microphone unit to detect a noise in a vehicle interior. Microphone units are disclosed in PTLs 4 and 5.